Hermetic seals which are suitable for use in vacuum glazing typically involve the use of solder glass, which is also referred to as glass frit. The term “solder glass” refers to a glass which melts and softens at a lower temperature than the glass sheets of the glazing, but which has a coefficient of thermal expansion which closely matches that of the glass sheets. As such, a suitable solder glass will depend upon the glass used for the glass sheets of the vacuum glazing. A typical example of glass used for vacuum glazing is soda lime glass.
The use of solder glass has the advantage that, unlike other solders such as metal solder, it is “compatible” with the glass sheets. For example, bonds between the solder glass and the glass sheets can be formed by inter-diffusion. At the same time, the solder glass is impermeable so that a low pressure within the internal volume of vacuum glazing can be maintained indefinitely for all practical purposes.
To form an hermetic seal between two glass sheets of a vacuum glazing, solder glass, normally in the form of a viscous liquid paste containing solder glass powder, is provided around the edges of the glass sheets, which are positioned in a spaced apart relationship, e.g. by way of support pillars placed between the sheets. The entire structure is then heated to a temperature at which the solder glass melts and whilst in its molten state, flows by capillary action between the spaced apart sheets and diffuses into the atomic structure of the respective glass surfaces, forming a strong and leak free joint between them.
Although the temperature at which the edge seal process occurs is less than that at which the glass sheets soften and melt, it is in general necessary for the temperature at which the sealing process occurs to be such that the glass sheets are quite close to the point at which softening and distortion occur. In other words the influence of the heating on the glass sheets cannot be ignored for all purposes.
As an example, the temperatures necessary to form the seal with the process described above can result in a significant relaxation of internal stresses in the glass sheets. Treatment of the glass sheets at such temperatures for that purpose is commonly referred to as annealing. Although the annealing may result in the removal of unwanted stresses in the glass sheets for some applications, for other applications the removal of residual stresses during the formation of the solder glass seal is undesirable. As an example, it is often required that the glass in windows and doors should be tempered or heat strengthened. Tempered glass contains internal stresses which need to be retained to maintain the increased strength of the glass sheet.
Since a significant stress relaxation in the glass sheets cannot be avoided during the forming of the hermetic edge seal made from solder glass as described above, it has up until now been impossible to manufacture vacuum glazing which incorporates an hermetic edge seal made from solder glass and which utilise (fully) heat strengthened glass.